In this longitudinal study in Wistar male rats, from weanling to senescence, we are exploring the effects of different ratio levels of dietary unsaturated/saturated fatty acids (U/S) on the life span and on several biochemical and ultrastructural parameters in brain, heart and liver. Animals are fed ad libitum six different diets resulting from the permutations of three different levels of U-S fatty acids (0.10; 1.5 and 13.5) and two levels of alpha-tocopherol (2 mg and 200 mg). The sequential ultrastructural studies of this project (at 3, 6, 12, 18, 24, 30, and 36 months) involve the stereological quantitative analysis of the fractional cytoplasmic volume of aging pigment (lipofuscin) in neurons, myocardial fibers and hepatocytes, the number and volume of nuclei, mitochondria, peroxisomes, residual bodies and rough and smooth endoplasmic reticulum. Biochemical determinations include serum levels of lipids and vitamin E and tissue contents of vitamin E, DNA, RNA, water, protein, lipids, glycogen, collagen, as well as the activities of a series of enzymes and lipoperoxidative potentials. This approach permits valuable comparisons and correlations between morphologic and biochemical parameters with age and with diet.